This invention relates to an electronic device and a method for making the same. More particularly, the invention relates to an electronic device wherein a semiconductor active layer and a gate electrode are in limited relative position from each other and wherein a space between conductor layers and a space between ohmic contact layers are, respectively, controlled, and also to a method for making such a device as mentioned above.
Existing electronic devices which are formed as a switching device relative to an active matrix substrate used in liquid crystal display devices and shutter arrays include reverse staggered thin film transistors (TFT) shown, for example, in FIGS. 3 to 5.
(1) With the structure of FIG. 3, both a semiconductor active layer 303 consisting of a-Si (i) and an ohmic contact layer 304 consisting of a-Si (n+) have a width greater than a gate electrode 301. When light is irradiated from the back side of a substrate, the light is permitted to invariably pass to part of the semiconductor active layer 303. Eventually, the OFF current (IOFF) rises with a lowering of S/N sensitivity.
(2) With the structure shown in FIG. 4, there is no problem involved in (1) above. However, a semiconductor active layer 403 consisting of a-Si (i) directly contacts, at the side face thereof, with a conductor layer 405 made, for example, of Al/Cr, thus presenting the problem that IOFF undesirably rises.
(3) With the structure of FIG. 5, the problems of (1) and (2) above can be solved. Nevertheless, an additional step is essential for forming a channel protective layer (i.e. an etch stopper layer) consisting of silicon nitride between a semiconductor active layer 503 and an ohmic contact layer 504.
As will be apparent from the above, in order to suppress the rise of IOFF in case where light is irradiated from the back side of the substrate, the prior art (3) can provide the best structure.
In this connection, the prior art techniques (1) and (2) have the following common problem.
(a) In order to form a source electrode and a drain electrode and also to remove the ohmic contact layer from the semiconductor active layer, a resist mask used to form the source and drain electrodes is employed to process the source and drain electrodes therewith. Thereafter, using this mask, the ohmic contact layer on the semiconductor active layer is processed. Accordingly, where the conductor layer and the ohmic contact layer are successively etched, the ohmic contact layer below the end of the conductor layer is likely to be side etched, thereby causing a gap to be formed. This leaves the residue of an etchant in the gap, thereby degrading TFT characteristics.
On the other hand, the prior art technique (3) also has the following problems.
(b) The semiconductor active layer is liable to suffer damages owing to the ion implantation used to form the ohmic contact layer.
(c) The production costs increase because of the additional step of forming the channel protective layer.
(d) Since the channel protective layer is additionally formed, the resultant device is formed as being more stepped, so that electric concentration or short-circuiting is apt to occur at the stepped portion or portions.
It is therefore an object of the invention to provide an electronic device which can suppress the rise of IOFF when light is irradiated from the back side of a substrate without the formation of any channel protective layer.
It is another object of the invention to provide a method for making such an electronic device at reduced production costs.
The above objects can be achieved, according to one embodiment of the invention, by an electronic device of the type which comprises a substrate, a gate electrode formed on one surface of the substrate, and a gate insulating film covering the substrate and the gate electrode therewith, characterized by comprising a semiconductor active layer formed above the gate electrode and having a width smaller than the gate electrode, and a source electrode and a drain electrode formed on the semiconductor active layer through an ohmic contact layer, the source electrode and the drain electrode being, respectively, made of a conductor layer and being in a spaced relation from each other, wherein the ohmic contact layers are, respectively, formed between the semiconductor active layer, and the source electrode and the drain electrode, the space between the source electrode and the drain electrode kept away from each other is wider than the space between the spaced ohmic contact layers, and the substrate is irradiated with light from the other surface on which the gate electrode is not formed.
Preferably, the semiconductor active layer of the electronic device of the invention is made of amorphous silicon, and the ohmic contact layer is made of the semiconductor active layer to which an impurity is added. The conductor layer should preferably be made of a metal such as Al, Ti, Mo or Cu, an alloy thereof, or a compound of the metal.
According to another embodiment of the invention, there is also provided a method for making an electronic device which comprises a substrate, a gate electrode formed on one surface of the substrate, and a gate insulating film covering the substrate and the gate electrode therewith, characterized in that where a semiconductor active layer having a width smaller than the gate electrode is formed just above the gate electrode, the gate insulating film and the semiconductor active layer are continuously formed by a CVD method in such a way that the semiconductor active layer is formed narrower in width than the gate electrode, and a source electrode and a drain electrode both made of a conductor layer are formed through an ohmic contact layer on the semiconductor active layer, that where the ohmic contact layer is formed between the semiconductor active layer and the source electrode and also between the semiconductor active layer and the drain layer, respectively, the ohmic contact layers, the source electrode and the drain layer are continuously formed by a sputtering method, that where a space between the source electrode and the drain electrode spaced from each other is wider than a space between the ohmic contact layers kept away from each other, the source electrode, the drain electrode and the ohmic contact layers are all continuously removed with an etchant, and that a light irradiating means is provided at the other surface of the substrate, where the gate electrode has not been formed, so that light is permitted to pass vertically to the substrate.
Preferably, according to the method of the invention, the etchant has an etching rate for the source electrode and also for the drain electrode higher than an etching rate for the ohmic contact layers.
In the first embodiment of the invention as claimed in claim 1, since the semiconductor active layer whose width is smaller than the gate electrode is formed just above the gate electrode, the rise of IOFF can be suppressed on irradiation of light from the other side of the substrate. Moreover, the source electrode and the drain electrode which are, respectively, made of the conductor layer and are formed on the semiconductor active layer through the ohmic contact layer, and the ohmic contact layers are formed between the semiconductor active layer and the source electrode and also between the semiconductor active layer and the drain electrode. By this, the rise of IOFF can be prevented.
In addition, the space between the source electrode and the drain electrode is wider than the space between the ohmic contact layers, so that when the multi-layer film consisting of the conductor layers and the ohmic contact layers is etched, the formation of a gap in the ohmic contact layers beneath the end portions of the conductor layers can be avoided.
In a preferred embodiment as claimed in claim 2, the semiconductor active layer is formed of amorphous silicon, so that this layer can be formed at low temperatures of not higher than 350xc2x0 C. This is beneficial for application to large-sized glass substrates.
In another preferred embodiment of the invention as claimed in claim 3, the ohmic contact layer is made of a semiconductor layer to which an impurity is added, and can be formed on a large-sized glass substrate, like amorphous silicon.
In further preferred embodiments of the invention, the conductor layer consists of a metal such as Al, Ti, Mo or Cu, or an alloy or compound thereof. If the space between the source and drain electrodes is wider than that of the ohmic contact layers, the ohmic contact layer and the conductor layers forming the source and drain electrodes can be continuously removed by the same etchant.
In another embodiment of the invention, since it is not necessary to form any channel protective layer, the number of steps can be reduced. Where the space between the source electrode and the drain electrode is formed as being wider than the space between the ohmic contact layers, they can be formed by continuously removing with the same etchant, making it unnecessary to use a plurality of masks and a plurality of etchants therefor. As a consequence, the number of steps can be reduced, enabling one to reduce the production costs.
In a further preferred embodiment, the etching rate of the etchant for the conductor layer of which the source and drain electrodes have been formed is higher than that for the ohmic contact layer. Accordingly, the space between the source electrode and the drain electrode kept away from each other can be formed as being wider than the space of the ohmic contact layers spaced from each other.